Code generation disallows spreading an interface or union against a union

[glasser]

In GraphQL, you should be able to spread an interface or union against a union, as long as there's some type that satisfies both. For example:

interface Error {
  message: String
}

type ErrorA implements Error {
  message: String
}

type ErrorB implements Error {
  message: String
}

type Success {
  updated: Int
}

union Result = Success | ErrorA | ErrorB

type Mutation {
  doIt: Result
}

mutation M {
  doIt {
    ... on Error {
      message
    }
    ... on Success {
      updated
    }
  }
}

This mutation is a valid operation, but graphql-client will block it with error: Failed to generate GraphQLQuery impl: The spread Result... on Error is not valid.

This is unfortunate as it's a nice pattern for mutations to return a union that allows a bunch of different error types, but to let a client process all errors together if they want.

(I ran into this while debugging an issue with https://github.com/apollographql/rover but I'm not personally a strong Rust developer so I wasn't able to quickly turn this into a standalone test case — if you can show the proper part of the repo to do so, I'm happy to do so.)